The invention relates to a seat for supporting the bottom area of a seated person, which seat is intended and adapted to form part of an article of seating furniture, such as a chair or a couch, a seat for a means of transport such as a car, a bus or an aircraft, or a wheelchair.
Such a seat is known in many embodiments and is usually applied in combination with a backrest.
The invention has for its object to embody a known seat such that the shear forces on the upper legs and buttocks are reduced and the pressure on the left and right tubera is decreased.
As a result of a non-uniform pressure distribution on the bottom area there occurs more load at some locations on this bottom area and less load at other locations. Seated persons who are more or less forced to sit for long periods hereby experience problems related to this sitting.
Sitting-related problems manifest themselves in, among other ways, pain in the bottom area, this sometimes even resulting in death of tissue or decubitus. Decubitus is the death of tissue due to poor blood circulation. The poor blood circulation is caused, among other reasons, by the fact that a relatively high pressure locally on and in the tissue reduces the flow through the blood vessels. Decubitus ulcers occur at those locations where bone structures lie close to the surface of the skin, such as in the case of the two left and right seat bones (tubera ischiadica) and the tail bone (os coccyx).
Unfortunately, existing products such as special cushions, so-called anti-decubitus cushions, still provide little or no improvement for such patients.
The invention has for its object to provide a solution for people affected by sitting problems, including decubitus.
The invention is based on the insight that a uniform load provides for a lower peak load, which in turn ensures less discomfort.
The seat according to the invention has for this purpose the feature that the seat consists of two parts, i.e. a left-hand seat part and a right-hand seat part, which seat parts are separated from each other along the nominal median plane of a user, each of the seat parts is pivotable around a pivot zone extending at least roughly perpendicularly of said median plane; and the projected position of the pivot zone lies in front of the tubera.
The seat according to the invention ensures that the upper legs and buttocks on the left and right are supported individually in uniform manner in accordance with a natural balance. The left-hand seat part and the right-hand seat part are each supported for individual pivoting by a pivot construction. This can for instance be a single pivot shaft shared by both seat parts, although other pivot constructions can also be used, for instance rod mechanisms or the like. It is not possible in the case of such mechanisms to refer to a single pivot axis, but there is a region around which the pivoting movement takes place. This region is referred to in this specification as “pivot zone”.
The description of the accompanying figures is now anticipated in the following.
The biomechanical model (FIG. 3b) of the seated human body assumed in the research by dr. ir. H. A. M. Staarink (page 31 of the book “Zo zit het!” ISBN 978 90 232 4341) teaches that a frictionless bottom area support is created at a seat angle φ of a minimum of 12° (FIG. 4). The position of the pivot shaft or pivot shafts respectively pivot zone or pivot zones is important for the distribution of the forces on the bottom area of the seated person. The seat angle φ is important in minimizing friction and shear forces. A support with low friction and shear forces is desirable. This can only be achieved with a correct choice of the seat angle.
A uniform load has a positive effect on the pressure-distributing properties of the seat. The seat parts can be freely movable and have for instance a rest position in which the front side of each of the seat parts occupies a highest position and the rear side a lowest position. Such a rest position can for instance be realized with suitable spring means. Use can also be made for this purpose of a specific location of the mass centre of the upper body of the seated person relative to each seat part.
According to an important aspect of the invention, the seat has the special feature that an angle measuring device is added to each of the seat parts with which the pivot positions of the seat parts can be measured and read, for instance a spirit level.
The seat can further be provided with pivot position adjusting means for adjusting the pivot positions of the seat parts.
According to yet another aspect of the invention, the seat has the special feature that the pivot position adjusting means are of mechanical, pneumatic or electrical type. Use can for instance be made of an electrical actuator.
The two latter variants have the advantage that they can be provided with fixation means for fixing at least one of the two seat parts, preferably both seat parts, in an adjusted pivot position.
In order to realize the best possible pressure distribution, the seat according to the invention can advantageously have the special feature that both seat parts have an anatomical form.
According to an important aspect of the invention, which also aims at a good pressure distribution, the seat has the special feature that both seat parts are provided with a pressure-distributing upper layer.
A further improvement in a uniform pressure distribution and an improvement in the sitting comfort is realized with an embodiment in which the pressure-distributing upper layer is air-permeable, is covered by a cover layer provided with perforations, and air pressure means, for instance fan means, are present which feed air under a certain pressure to the pressure-distributing upper layer, this air leaving the upper layer via the perforations present in the cover layer. This embodiment has the further advantage of realizing a certain ventilation and cooling of the skin and the tissues thereunder.
According to yet another aspect of the invention, the seat can comprise: control means for controlling the pivot position adjusting means such that the pivot positions of the seat parts vary through time such that the pressure loads on the tubera vary through time.
It can be of great importance that the left-hand seat part can be adjusted independently of the right-hand seat part. A difference in lower leg length or differences in thickness of for instance shoe soles on the left and right adversely affect the pressure-distributing results in the case where the seat, other than in the invention, were not divided.
In the case a footrest is set too high (FIG. 3c) the upper leg will be supported less, or even not be supported, and the buttock must absorb the sitting load, whereby the danger of decubitus will increase. If the footrest is adjusted too low (FIG. 3a) the upper leg, and particularly the part behind the knee, is loaded too much, resulting in an increased risk of restriction of blood vessels and nerve-paths, with all the adverse consequences this entails. The invention evens out the possible difference between left and right almost completely, and therefore causes considerably less high sitting load peaks (FIG. 3b). The pivoting of the seat parts (FIG. 6) is essential in providing a correct support and guaranteeing a balanced pressure distribution. The operation of the seat parts could be compared to the operation of a pair of scales.
Adjustment of the seat angles (φL (left) and (φR (right) (see FIG. 6) is brought about by adjusting the heights of the left and right-hand footrests. Integrated seat angle measuring devices measure the (φL and the (φR relative to the horizontal plane and give qualitative and/or quantitative feedback to the seated person and/or to the therapist.
Many seat cushions are known which claim to cause no friction or shear forces but, if there is no relation between a seat cushion and the seat angle (φ) and the functional backrest angle (α+φ) (see figure A), friction and shear forces cannot be precluded.
During use the seat according to the invention forms part of a full seat support comprising the seat and a backrest, and optionally a footrest.
The seat according to the invention can be applied as replacement for an existing seat or be applied integrally in combination with a back support in for instance a car seat, office chair, garden seat or a wheelchair.
Seat which may or may not be embodied with a backrest are per se known. Also known are seats which can pivot in different ways.
Described in WO-A-1995/015101 is a total seat support comprising a seat wherein the seat can pivot as a whole. A similar structure is specified in WO-A-2005/0116527, although here the seat cannot pivot around a fixed pivot axis.
Described in U.S. Pat. No. 5,580,128 is a segmented seat wherein there are two pivot axes transversely of said median plane.
It is known from the literature that discomfort is also caused by friction forces on and shear forces in the tissue. Different researchers assume that the influence of friction and shear forces on discomfort, as manifested for instance in decubitus, could be greater than the influence resulting from perpendicular load. Friction and shear forces are caused largely by an incorrect value of the seat angle φ.
Research has shown that an anatomically correct torso support starts at a functional backrest angle (α+φ) of about 115° (figure A) and ends at an angle of about 123°. At a larger angle (α+φ) the head must always be actively supported, and this is therefore no longer an active sitting posture, but the beginning of the lying posture.
If in the model of the human body of FIG. 5a masses are assigned in accordance with anthropometric data to the body parts assumed in the model, it then follows from a (bio) mechanical analysis that, at a functional backrest angle of (α+φ) of 115° and a seat angle φ of about 12°, no friction forces occur on the bottom area as a result of the external load.
If the seat angle (α+φ) increases, angle φ must also increase in order that no shear forces are caused in the seat as a result of the external load. There is a direct relation between a seat angle (α+φ) and seat angle φ. If both seat parts are provided with an angle indicator, the measured angles φL and φR are indicators of whether or not friction and shear forces occur on the seat parts.
It is important to stimulate awareness of posture through interaction of the seat and the user by means of said angle indicators added to both the left and the right-hand seat part. The user and/or the therapist can check the seat angle by reading the value of φL and φR from the integrated angle indicators.
People with sitting problems are often consciously involved in their rehabilitation. In order to stimulate this process the invention provides users, by means of the seat angle indicator, with an aid allowing them to monitor their own sitting posture and, if necessary, to adjust it themselves if possible.
The seat does not require an actuator to position a user in the correct posture because the user balances him/herself using the balance in the support, provided the footrest is set to the correct height.
An actuator can however be added for a forced alternating load between the left, right, front and rear sides of the seat. This provides the option of varying the load on both tubera (FIG. 7).
By tilting the left-hand supporting surface further forward (reducing angle φL) the left tuber will be loaded more than the right tuber. By varying the left and right seat angle (respectively φL and φR) the loads on the tubera will begin to vary, this enhancing flow of blood and moisture in the surrounding tissue. The risk of decubitus can thus be reduced in preventive manner.
The transfer, i.e. sitting down in or standing up from the seat, is facilitated in that the seat parts pivot along with the displacement of the centre of gravity of the user.
In addition to posture and pressure distribution, temperature and moisture are also important factors in the development of decubitus or discomfort. In the invention the seat contact surfaces are provided with a cover and a pressure-distributing layer which are air and moisture-permeable so as to bring about a decrease in the air humidity and temperature on the seat surface, optionally brought about by a forced airflow. FIG. 8 makes clear that forced ventilation in the pressure-distributing layers of the support is realized by means of an electrically driven fan.